A political spring is an abrupt, broad, sustained increase in public dissent in a state that has prohibited it, as in Czechoslovakia in 1968 or Tunisia in early 2011. Some springs produce offspring – clusters of events within neighbouring states (civic unrest, increased state repression, co-option of dissent, revolution) and among those states (intensification of international rivalries, foreign interventions). An English Spring in 1558–9 produced such a cluster in Northwestern Europe. This article addresses the underlying causal mechanism connecting springs and their offspring, rather than the related correlational question (viz. under what conditions a spring is followed by offspring). That mechanism is transnational group polarisation, or the progressive separation of preferences across a population into pro- and anti-government groups. Transnational polarisation along a pro-versus-anti-government axis is an endogenous process triggered by exogenous events, such as violence or public demonstrations that raise the status of, or threat to, one of the groups. It presents powerful actors across states with new threats and opportunities and can help explain how the Tunisian Spring of early 2011 produced throughout the Arab Middle East infectious unrest, serial repressions and reforms, heightened international tensions, and foreign interventions.